1. Field of the Invention
This invention relates to liquid toners that are useful for electrographic and electrophotographic processes.
2. Discussion of the Art
A general discussion of color electrophotography is presented in "Electrophotography," by R. M. Schaffert, Focal Press, London & New York, 1975, pp. 178-190.
Electrophotographic systems are systems in which a toner is deposited on a charged surface and subsequently transferred to a receiving sheet. Electrophotographic systems employing liquid toners are well known in the imaging art. See, for example, Schmidt, S. P.; Larson, J. R.; Bhattacharya, R. in Handbook of Imaging Materials, Diamond, A. S., Ed.: Marcel Dekker, New York, 1991, pp. 227-252 or Lehmbeck, D. R. in Neblette's Handbook of Photography and Reprograph, Sturge, J., Ed.: Van Nostrand Reinhold, New York, 1977, Chapter 13, pp. 331-387. A liquid toner is a dispersion of colloidal particles in a dispersing medium having a low dielectric constant. The particles comprise a pigment and a film-forming resin and carry an electrostatic charge. The particles in the dispersion are capable of migrating under the influence of an electric field and being deposited on a surface bearing an opposite charge, thereby forming an image.
In most instances, the preferred dispersing medium has been a high boiling hydrocarbon that has both a low dielectric constant (e.g., less than 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoconductor drum, transfer belt, and/or receptor sheet. An example of such a preferred solvent that is commercially available is the family of solvents having the trade designation "ISOPAR" (boiling point range: 130.degree.-160.degree. C.). Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image.
Both dyes and pigments have been used as the colorant in toners for electrophotography. One of the advantages of pigments is that migration, or "bleeding", is minimized at the fusion step. The primary advantages of dyes are their bright colors and transparency. Polymeric dyes offer the advantages of being both highly transparent and non-migratory, which make them well-suited in applications requiring high quality images, such as, for example, in proofing or business graphics. Another advantage of using polymeric dyes instead of pigments is that the polymeric dyes have the potential to be more conformable to the final image receiving layer. Conformability is particularly important when the final image receiving layer is plain paper, where abrasion resistance and adhesion are considerations. A further advantage of toners containing polymeric dye over toners containing pigments is greater stability of the dispersion, because flocculation, caused by desorption of the colorant from the toner, is not possible with polymeric dyes. A process advantage of toners using polymeric dyes is that the milling operation required to incorporate a pigment in a dispersion medium is avoided.
Polymeric dyes can be classified in two classes: (1) backbone polymeric dyes and (2) pendent polymeric dyes. In backbone polymeric dyes, the chromophore is a segment in the polymeric chain. A schematic diagram of a backbone polymeric dye is shown below: ##STR1## where D represents a chromophore. In pendent polymeric dyes, the chromophore is tethered to the polymeric chain either directly via the chromophore or indirectly via a connecting group, e.g., alkylene group. A schematic diagram of a pendent polymeric dye is shown below: ##STR2## where D represents a chromophore. A typical backbone polymeric dye can be prepared by reacting a dye containing two reactive groups, such as two acid chloride groups, with a colorless organic diol or diamine. Many backbone polymeric dyes are based on polyesters or polyamides. There are two general methods for preparing pendent polymeric dyes: (1) the polymerization of a monomer containing a pendent chromophore, and (2) the reaction of a pre-formed polymer with a reactive dye or reactive dye developer. Typically, dyes produced by method (1) are made by copolymerization of a free radically polymerizable monomer having a chromophore and a colorless free radically polymerizable monomer. Co-monomers are often incorporated to modify solubility or dispersibility properties of the resultant polymer.